Crop diseases in Louisiana

Last year, ample amounts of leaf rust were found in Louisiana’s wheat crop. In response, Boyd Padgett and colleagues have been conducting fungicide and variety screening.

“We look at applications put out at flag leaf emergence and also at 50 percent heading,” the LSU AgCenter plant pathologist said at the recent Northeast Research Station field day outside St. Joe, La. “Growers ask whether a fungicide should be used. What we’ve found is the application window isn’t as critical for leaf rust as it is for stripe rust.”

Stripe rust first became prominent in northeast Louisiana around 2000. It has since become “our number one disease. When I was in graduate school, it wasn’t much of a problem. It’s kind of funny, but as soon as I developed research programs on the disease, its severity dropped. But that doesn’t lessen the need for growers to properly time fungicide applications.”

Padgett is also focused on new products available this year for managing rust diseases in wheat.

“In a nutshell, we found the premix fungicides — those that contain two modes of action — are most effective. The beauty of having two modes of action is it provides effective resistance management. Hopefully, these fungicides will be around for a long time.”

Last season, when the price of wheat rose, farmers became interested in making a prophylactic fungicide application to disease-resistant varieties.

For the last three years, “we’ve been doing work on this station and the Macon Ridge station where we’ve studied wheat varieties developed by Steve Harrison (LSU AgCenter wheat breeder). Those varieties have very good genetic resistance to disease. You can put these varieties out and aren’t affected by rust diseases as well as some of the other diseases.

“Fungicides were sprayed on these varieties and, to no surprise, there was no response. That information should help producers save an application — as much as $20 (per acre).”

Corn

This year, there has been “quite a bit of common rust — more than I’ve seen in the past” on Louisiana’s corn crop, said Padgett while holding up several afflicted plants. “It’s kind of a cinnamon brown pustule and is very dramatic. I saw it in fields before tasseling.”

When the disease shows up, Padgett is frequently asked if a fungicide needs to be put out. Six or seven years ago, “fungicide applications in corn were few and far between. People just didn’t do that. More recently, that has changed and the AgCenter has been doing work to find out if fungicides are justified.”

Once temperatures reach about 75 degrees — “and it’s been well above that for the last couple of weeks” — common rust doesn’t thrive. In that respect, it’s similar to Asian soybean rust.

“If you look at common rust under a hand lens, the pustules are all dried up. So, in my opinion, most of the time growers won’t need a fungicide for common rust in corn.”

Southern rust, however, is a different story.

“Thankfully, most of the time southern rust epidemics get started very late in the year — usually so late they don’t have time to impact yield.

“We have ongoing studies on corn fungicides here and at the Macon Ridge station. Six to eight popular varieties are in the tests. Those have been sprayed — usually with Headline or Quilt — and compared to a block of the same varieties that haven’t been sprayed. Those have then been compared for yield and stalk density. So, it may prove there isn’t a yield increase but will prevent lodging.”

In Padgett’s studies, the most consistent thing found is inconsistent response. Currently, “the AgCenter isn’t recommending a (blanket) fungicide application on corn. There may be certain scenarios where a fungicide is justified.”

Padgett also pointed to Midwest-based work on fungicides and corn. “Purdue (University researchers) made applications prior to tasseling. Fungicides usually go out at tasseling. But in studies where a crop oil or surfactant is used, they’ve found if you make a fungicide application too early it can result in arrested ear development — the ear won’t elongate, resulting in a yield decrease.

“So, if you decide to make a fungicide application, be sure your corn is at full tassel. And for those who will make the applications regardless, I discourage the use of any type of adjuvant.”

Soybeans

Padgett’s research in soybean has escalated since the discovery of soybean rust in Louisiana a few years ago. (For more, see http://deltafarmpress.com/soybeans/soy-rust-0616/index.html.) As of mid-June, soybean rust is limited to the southern part of Louisiana, mainly in the coastal parishes. It has been found on soybeans and kudzu.

“I’ve been getting a bunch of calls from consultants asking if there’s a need for a fungicide application. That’s an ongoing discussion amongst researchers.

“Related to this is a common question I’m asked: what growth stage is my soybean crop in? When I was in graduate school working with determinate varieties, it was pretty easy to say what growth stage a plant was in. There was uniform pod production across the plant. With Group 4 beans, that isn’t the case.”

To make a determination on a Group 4 variety’s growth stage, “you count down four nodes from the very top. At the bottom of this particular plant is a full pod with a seed in it. Someone would say, ‘Well, I’m between R-5 and R-6 (R-5 has a small seed, R-6 has a full seed).’ But this plant is actually still not quite at R-4. It’s very important to know the crop’s actual growth stage before making management decisions.

“To manage disease, good genetic resistance is the way to go. Fungicides are the second option.”

Cotton

Most of Padgett’s research with cotton is focused on seed treatments. “Some of them do have a fit in our agricultural landscape. The seed treatments usually don’t have the residual activity of in-furrow products.

“Dynasty, Trilex — Valent is also coming out with a fungicide seed treatment — work. However, they don’t last for 9,000 days. Realize that the seed treatments will probably provide a couple of weeks of residual activity.”